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RESEARCH REPORTCFS-NEES Building Structural Design NarrativeR.L. Madsen, N. Nakata, B.W. SchaferCFS-NEES - RR01October 2011

This report was prepared as part of the U.S. National Science Foundation sponsored CFS-NEES project:NSF-CMMI-1041578: NEESR-CR: Enabling Performance-Based Seismic Design of Multi-Story ColdFormed Steel Structures. The project also received supplementary support and funding from the AmericanIron and Steel Institute. Project updates are available at www.ce.jhu.edu/cfsnees. Any opinions, findings,and conclusions or recommendations expressed in this publication are those of the author(s) and do notnecessarily reflect the views of the National Science Foundation, nor the American Iron and SteelInstitute.Authors:R.L. Madsen, Senior Project Engineer, Devco Engineering, Corvallis, OR, USA.N. Nakata, Assistant Professor, Department of Civil Engineering, Johns Hopkins University, Baltimore,MD, USA.B.W. Schafer, Swirnow Family Faculty Scholar, Professor and Chair, Department of Civil Engineering,Johns Hopkins University, Baltimore, MD, USA.www.ce.jhu.edu/cfsnees 20112

CFS-NEES Building Structural Design NarrativeINTRODUCTIONThe NSF sponsored CFS-NEES1 project R-CR: Enabling Performance-Based Seismic Designof Multi-Story Cold-Formed Steel Structures project was undertaken to study the behavior,particularly seismic behavior, of light-framed structures using cold-formed steel cee-sections asthe primary gravity load carrying elements with wood structural panel diaphragms andshearwalls as the primary lateral load resisting system.Devco Engineering, Inc. was selected to develop design calculations and drawings for thestructure based on criteria determined by the research team. Input on the details of design wasalso sought from industry professionals through the Industry Advisory Board (IAB)2. The detailsdeveloped in the design phase will be studied via component and full-scale shake table testingof the structure.This report discusses the design of the gravity and lateral systems for the CFS-NEES building.Specific calculations and drawings are attached herewith as appendices for reference.Design CriteriaDesign of the structure was based on a site in Orange County, California. Gravity and lateralloads were determined per the 2009 edition of the International Building Code (IBC) based onthis location.For member sizing, the “North American Specification for the Design of Cold-Formed SteelStructural Members”, 2007 edition (AISI S100-07) was used. Member callouts were based onSSMA/SFIA criteria. Shearwall and diaphragm design was based on the “North AmericanStandard for Cold-Formed Steel Framing – Lateral Design”, 2007 edition (AISI S213-07).Wind and seismic forces were determined based on a location at 520 W. Walnut Blvd, Orange,California (latitude 33.8 degrees; longitude -117.86 degrees).For simplicity, and consistent with industry standards, allowable strength design (ASD) wasused for members and connections not part of the lateral force resisting system (LFRS). Fordesign of the LFRS, load and resistance factor design (LRFD) was used.Architectural ConceptThe architectural concept for the CFS-NEES building was developed by the project team. SeeAppendix 6 for a rendition of the architectural concept.12See www.ce.jhu.edu/cfsnees for detailsSee www.ce.jhu.edu/cfsnees/advisoryboard.php for member list3

Calculation Systems and NotationsCalculations were developed using standards employed at Devco Engineering for pagenumbering and labeling of attached documents. The following describes the system used:The particular element design being undertaken is double underlined at the top of the first pageassociated with the design of that element. The criteria used to size the element, for exampleloading, span lengths and any special considerations follow. Final member or connectionselection is double underlined with an arrow on the right hand side of the page.Computer printouts or other associated documents related to a specific element design areattached behind the hand calculations for that element. These supplemental documents arereferenced by a number inside a hexagon on the hand calculations and the same symbol andnumber can be found in the upper right hand corner of the related printout.Where spreadsheet printouts are provided in the appendices, black values are labels, bluevalues are user inputs and red values are calculated within the spreadsheet.SoftwareThe following software was used in the development of the calculations: AISIWIN version 8, Devco Software, Inc. Used for member sizing of simple spanmembers with uniform loads and axial loads were applicable.LGBEAMER version 8, Devco Software, Inc. Used for member sizing of more complexspan and load conditions.Microsoft Excel: Used to develop spreadsheets for lateral analysis and other generalpurpose calculation tasks.Member NomenclatureMember designations were used per SSMA/SFIA standards.AppendicesAppendices 1-5 attached contain the following:Appendix 1: Framing Member DesignAppendix 2: Seismic Lateral AnalysisAppendix 3: Shearwall and Diaphragm Analysis and DesignAppendix 4: Lateral System Design – Supplemental CalculationsAppendix 5: Design Drawings dated 10/31/11Appendix 6: Architectural concept drawings4

Structural Design SummaryGravity SystemBased on input from the IAB, a ‘ledger framing’ system was chosen rather than traditionalplatform framing. According to the IAB, ledger framing which attaches floor and roof joists to theinside flanges of the load-bearing studs via a combination of track and clip angles is currentlythe dominant method of construction. Studs are broken at the top of each floor level and cappedwith a track. Walls above are stacked on the lower wall top track. See Appendix 5, details 1 and2/SF4.40.Roof JoistsRoof joists were designed as simple span members with uniform loading. End rigidity of theattachment to the stud walls was not considered in the roof joist design. Design loads included20 psf dead load, 20 psf live load and wind uplift per IBC requirements. Note that for theeffective wind area associated with the joist spans for this building, maximum corner wind upliftwas calculated at 14.1 psf and thus was not a significant concern in the design.Roof joist deflection was limited to L/240 for dead load and L/180 for total loads. For distortionalbuckling, k was conservatively taken as zero. Had additional flexural strength been required,the k value appropriate for the joists selected and OSB sheathing on the compression flangecould have been used.Based on these loads and a maximum clear span of 22 feet, 1200S200-54 joists at 24 inches oncenter were selected. The compression flange of the joists was considered to be continuouslybraced via attachment of sheathing. In accordance with industry standards, two rows of bridgingwere specified in order to minimize joist rotation.Because the web height-to-thickness for the selected joists exceeded 200, web stiffeners wererequired at member ends. Stiffening was accomplished with clip angles screwed to the joist andto the rim (ledger) track. This method transfers the reaction from the joist web to the support indirect shear rather than bearing, thus precluding web crippling failure in the joists.Rooftop mechanical units each weighing up to 600 lb were anticipated. Design of the joists forsupport of these units was based on the load being distributed to at least two joists with two 150lb point loads supported by any individual member. Based on these loads, back-to-back1200S200-54 joists were specified at mechanical unit supports.Roof joist design, including sizing of joists at mechanical units and connection of joists toexterior walls can be found in Appendix 1, page R-1. Drawings related to roof joists can befound in Appendix 5, sheets SF4.02, SF4.20 and SF4.40.Floor JoistsIn addition to the standard 18 psf dead load to account for framing, sheathing, flooring and thelike, a 15 psf partition load was included to account for partitions that may be moved at varioustimes during the structure’s life span. Live load for floor joist design varies by location. Forexample, the typical live load is 50 psf but 80 psf is required at corridors. As such, joists were5

designed as simple span members with varying distributed loads. Similar to the roof joists, endrigidity of the connection to the wall was not considered.Deflection limits of L/240 for total loads and L/360 for live loads were used. For distortionalbuckling, k was conservatively taken as zero. Had additional flexural strength been required,the k value appropriate for the joists selected and plywood sheathing on the compressionflange could have been used.Based on the above, 1200S250-97 joists 24 inches on center were selected. The compressionflange of the joists was considered to be continuously braced via attachment of sheathing. Tworows of bridging were specified in order to minimize joist rotation. In addition, due to the highend reactions and relatively short bearing length, web stiffeners were required at joist ends.Stiffening was accomplished in the same way as at the roof, but with additional fastenersrequired for the higher loads.At the clerestory opening, single track headers were designed to carry floor joist loads to carrierjoists on either side of the opening. A 1200T200-68 was chosen for the 8’6” span. Carrier joistswere designed for a distributed load equal to one half of that used at typical joists in combinationwith the concentrated loads from the headers on each side of the opening. Single 1200S350-97carriers were selected.Floor joist analysis and design is found in Appendix 1, pages F-1 and F-2. Drawings for floorjoists can be found in Appendix 5, sheets SF4.01, SF4.20 and SF4.40.Load-bearing WallsFor a desired clear height of framing of 8’0” and 12” deep joists, studs were designed as 9 ft. inlength. Code prescribed wind loads, when reduced for area, were less than 15 psf. As such, aslightly conservative value of 15 psf wind load was used for stud design.Studs above the 2nd floor platform were designed to carry wind load in combination with roofdead and live loads. Load combinations per ASCE 7-05 were used. The total gravity load of440 lb/stud was used based on the roof joist reactions. Gravity loads were applied at theinboard stud flange, resulting in an end eccentricity of 3 inches to the center of the studs. Sincewalls will receive gypsum board sheathing on at least one flange, k for distortional buckling wastaken as zero per CFSEI Technical Note G100-08. Based on these criteria, 600S162-33 studsat 24 inches on center were chosen. The studs were acceptable with either sheathing bracing,or discrete bracing near mid-height. Since some tests may be performed without interiorsheathing, discrete bridging (noted as CRC, or cold-rolled channel in the calculations) will berequired for these tests.With the stud size known, the connection of the roof joists to the wall was designed. Theconnection was designed for shear due to gravity loads plus tension due to outward acting windloads (suction) on the wall studs.In order to allow the roof diaphragm to extend over the top of the level 2 walls, the parapet wasdesigned as a free-standing cantilever. Track and fasteners were chosen to resist theassociated overturning forces.6

Walls running perpendicular to the joists transfer out of plane lateral forces to the diaphragm viatheir connection to the joists. However, for walls parallel to the joists, transferring out of planewall forces into the diaphragm is accomplished via a direct connection of the wall to thediaphragm sheathing. For plywood to steel connections, allowable screw forces were based onthe American Plywood Association publication APA E830D “Technical Note: Fastener Loads forPlywood – Screws”, dated August 2005.Lower level walls were designed similarly to the upper level walls except that in addition to roofgravity loads, floor gravity loads were also considered. Gravity loads from the roof and wallabove were considered concentric. Gravity loads from floor joists were applied at the inboardstud flange, thus introducing an eccentricity of half the stud width or 3 inches. On this basis,600S162-54 studs @ 24 inches on center with discrete bridging at mid-height were chosen.With the stud size known, the connection of the floor joists to the wall was designed. Theconnection was designed for shear due to gravity loads plus tension due to outward acting windloads (suction) on the wall studs.At the stair clerestory the carrier joists apply concentrated vertical loads to the 1st floor wallstuds. Based on the maximum load from the carrier joists and from the roof and wall above, itwas determined that two 600S162-54 studs would be required along with additional fastenersfrom the rim track to the studs.At the northwest exit stair, the 2nd floor joists are supported by an interior wall. The interior wallis subjected only to 5 psf partition pressure and does not support roof gravity loads. Accordingly,these studs were sized as 362S162-54 at 24 inches on center with bridging at 48 inches oncenter.Additionally at the northwest exit stair, the exterior wall studs span the full 18’ 0” height to theroof joists. These studs support only roof gravity loads. On this basis, the studs were sized as600S162-54 at 24 inches on center with bridging at 48 inches on center.Design of structural walls can be found in Appendix 1, pages W-1 through W-5. Drawingsdepicting the load-bearing walls can be found in Appendix 5, sheets SF4.20, SF4.30 andSF4.40.2nd Floor Wall OpeningsTo support loads around window and door openings, headers, sill and jambs were sized. Amaximum opening width of 8’ 0” was considered. For windows, openings were considered to be4’ 0” tall with a sill height of 2’ 6”.For openings at the 2nd floor, the perimeter rim track or joists were found to have sufficientcapacity to carry gravity loads over the opening. As such, no additional gravity header wasspecified.Header and sill tracks were sized as 600T150-33 to carry a 15 psf lateral load from jamb-tojamb. The connection of these members to the jamb studs was designed to support 196 lb of7

lateral shear. Per AISI S100-07 section E4, The shear capacity of a #10 sheet metal screw in33-mil steel is 177 lb/screw. As such, (4) #10 as specified is, by observation, adequate.Jamb studs were sized based on the lateral reactions from the header and sill as well as theeccentric vertical reaction from the rim track or joist above. To account for the eccentric natureof gravity loads, a moment couple was included based on 3 inches of eccentricity and a 12 inchdeep member. An option for using two 600S162-33 or a single 600S162-54 jamb was provided.Interconnection of the two-member configuration was designed per AISI S100-07 D1.2.Design of the jamb/rim track connection considered the concentrated shear due to gravity loadsas well as the top of jamb lateral reaction from the jamb analysis. Screw quantity wasdetermined based on minimum 33-mil jambs.Design of openings in the 2nd floor walls can be found in Appendix 1, pages W-6 and W-7.Framed opening drawings can be found in Appendix 5, sheet SF4.50.1st Floor Wall OpeningsFor the long side of the structure, the 1200T200-97 rim track above openings was analyzed andfound to be sufficient to carry gravity loads over openings up to 6’ 6” in width. For largeropenings, two 1200S250-97 were specified. The two 1200S250-97 header members were alsospecified for openings where clerestory carriers were supported.For the short side of the structure, the maximum opening was 6’ 0” in width. As such, the1200S250-97 end joist could easily carry the gravity loads over the opening.Head and sill tracks were sized as 600T150-54 for 15 psf lateral pressures.Jambs were designed with considerations similar to those at the 2nd level, but with additionalgravity loads from the structure above. On this basis, an option for two 600S162-54 or a single600S200-68 were specified.For large openings where gravity loads were exceptionally high, rather than rely on the screwshear to support the entire gravity loads, trimmer studs (studs immediately below the headerthat support header gravity loads as axial loads) were designed to provide a bearing typesupport for the header. 600S162-54 trimmers in combination with 600S162-54 king, or jambstuds were specified.Design of openings in the 2nd floor walls can be found in Appendix 1, pages W-8 through W-10.Framed opening drawings can be found in Appendix 5, sheet SF4.50.Lateral SystemBecause testing will be based on shake-table simulated seismic forces, the design of the lateralsystem focused on seismic design.Lateral forces were determined based on mapped short period spectral response accelerationparameter, Ss, and mapped 1-second spectral response acceleration parameter, S1 for thelocation described previously. Site Class D was chosen as is typical for sites in the vicinity ofthis project. For the office occupancy chosen, IE 1.0 was used.8

Lateral resistance was provided by wood structural panel shearwalls. For this system, thefollowing parameters were derived from ASCE 7-05 Table 12.2-1:Response Modification Coefficient, R 6.5Overstrength Factor, 0 3Deflection Amplification Factor Cd 4The resulting base shear coefficient was calculated as Cs 0.143.The effective seismic weight, W used in ASCE 7-05 Eq’n 12.8-1 was based on estimatedweights of roof, floor and exterior walls. A 1200 lb allowance for roof top MEP was included. Inaddition, per ASCE 7-05 section 12.7.2, a 10 psf allowance for partitions was included on the2nd floor. Reduced seismic weight due to stair openings in the 2nd floor were not considered asthe weight of attached stair elements would likely counteract any reduction in floor mass. A totalseismic weight of approximately 78 kips was determined; resulting in a seismic base shear forceof approximately 11 kips.The vertical distribution of the calculated shear was based on ASCE 7-05 section 12.8.3. Thedesign shear forces at the roof and 2nd levels were determined to be roughly 6.5 and 4.5 kipsrespectively.Calculation of Cs, W and the seismic shear at each level is shown in Appendix 2, page 1 andAppendix 1, sheet L-2.ShearwallsBased on the proposed location of windows and doors, shearwall locations were selected oneach of the (4) perimeter walls. Both Type I and Type II shearwalls were investigated. However,for this structure, the Type II shearwalls did not, in the opinion of the investigators and the IAB,provide a significant benefit. As such, Type I shearwalls were selected throughout.The size and location of shearwalls on each side of the building varied. As such, the horizontaldistribution of shear was determined based on an estimate of shearwall stiffness. Shearwallstiffness was estimated based on AISI S213-07 Eq’n C2.1.1. Spreadsheets were developed toallow interactive design of the shearwall with changing stiffness. See Appendix 3, sheet 1 forcalculation of horizontal shear distribution.Based on the force distribution, shearwalls were selected per the procedures of AISI S213-07.OSB sheathing was selected on the basis of economy of OSB and on the fact that for 54-miland heavier framing, a fixed maximum aspect ratio of 2:1 applies to Structural 1 sheathing butnot to OSB. The typical 2nd floor stud framing was specified as 33-mil, but in order to meetstrength requirements 54-mil chord studs were selected. Also minimum 43-mil top and bottomtrack were specified. Therefore, shear values applicable to 43 or 54-mil framing members wereused. Per Table C2.1-3 of AISI S213-07, for edge fasteners at 6 inches on center, the nominalshear strength of the assembly selected was 825 lb/ft. Analysis of the individual shearwalls isfound in Appendix 3, sheet 2.9

ASCE 7-05 Table 12.12-1 limits seismic story drift to 0.025hsx for the type of structurecontemplated where hsx is the story height. Drift wa